Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
MIT App Inventor - App Inventor is a cloud-based tool, which means you can create apps for phones or tablets right in your web browser.
Invision - Prototyping and collaboration for design teams
Bubble.io - Building tech is slow and expensive. Bubble is the most powerful no-code platform for creating digital products.
Zeplin - Collaboration app for UI designers & frontend developers